(1) Field of the Invention
This invention relates to X-ray CT (Computer Tomography) apparatus for use in medical and industrial fields to scan an object with X-ray beams and obtain three-dimensional images of the interior of the object, and particularly to X-ray CT apparatus for carrying out helical scans.
(2) Description of the Related Art
An X-ray CT apparatus irradiates an object under examination with X-ray beams from all directions through 360.degree. in each plane (slice plane) of the object, and acquires projection data for the respective directions through 360.degree. by detecting intensity of transmitted X rays. Then, the apparatus puts the projection data to a reverse projection to reconstruct a distribution of X-ray absorption rates (sectional image) on the slice plane. In order to acquire projection data in the directions through 360.degree. by means of X-ray beams, an X-ray tube usually is revolved round the object to describe a circular path on the slice plane, and the X-ray beams generated by the X-ray tube are directed toward the axis of revolution. The object is placed adjacent the axis of revolution, and an X-ray detector opposed to the X-ray tube across the object detects X-ray beams transmitted through the object. In this way, the above slice plane of the object is scanned by X-ray beams to provide projection data. The X-ray detector may include a plurality of detecting elements arranged in the form of an arc. This X-ray detector is revolved with the X-ray tube while maintaining the mutually opposed relationship across the object. Alternatively, the X-ray detector may include numerous detecting elements fixedly arranged on an entire circumference.
It is desired from the point of view of improved X-ray use efficiency to arrange the detecting elements of the X-ray detector two-dimensionally including a direction perpendicular to the slice plane as well. A specific example of X-ray CT apparatus commercially available today includes detecting elements arranged in two rows to collect data for two slices simultaneously. Further, an X-ray CT apparatus has been manufactured for trial, which employs an X-ray image intensifier as X-ray detector for continuously detecting X-ray incidence positions in the direction of a slice plane as well as the direction perpendicular thereto.
In addition, an X-ray CT apparatus of what is known as the helical scan type has been developed recently. This apparatus moves an object in a direction perpendicular to slice planes thereof while revolving an X-ray tube to scan the object with X-ray beams emitted from circumferential positions. As a result, the X-ray beams scan not only a single slice plane as above, but helically scan a three-dimensional space having a width in the direction perpendicular to the slice plane. The data thereby collected are put to a reverse projection in a helical mode to reconstruct a three-dimensional image of a section of the object.
FIG. 1 shows a helical scan type X-ray CT apparatus employing an X-ray detector having detecting elements in a two-dimensional arrangement. As seen in FIG. 1, a gantry 10 houses a single-focus X-ray tube 17 and an X-ray detector 12, and defines a tunnel (not shown) centrally thereof. The X-ray tube 17 and X-ray detector 12 are supported by a rotary support, not shown, to be revolvable together while maintaining a mutually opposed relationship across the tunnel. In this example, the X-ray detector 12 includes five rows of X-ray detecting elements juxtaposed in the direction perpendicular to a plane defined by a revolving circumference. A patient 41 is placed on a top board of a bed 42, and the top board is movable (sideways in FIG. 1) to introduce the patient 41 into the tunnel of gantry 10.
Assuming that a plane (slice plane) in which the X-ray tube 17 and X-ray detector 12 are revolvable is X-Y plane, and that the direction perpendicular to that plane (along the axis of revolution) is Z direction, this Z direction corresponds to the axial direction of patient 41 in which the patient 41 is introduced. When the patient 41 is moved in Z direction during a revolution of X-ray tube 17 and X-ray detector 12, a helical scan of the patient 14 is effected by X-ray beams emitted from the X-ray tube 17 as shown in FIG. 2. This provides X-ray transmission data relating not only to one slice plane perpendicular to the body axis of patient 41 but to an entire solid having a thickness in the axial direction. The pitch of this helical scan cannot be as large (coarse) as an entire width D (in Z direction) of X-ray detector 12 as shown in FIG. 1, but has to be about the same as width "d" of one detecting element as shown in FIG. 2. A three-dimensional image of the patient 41 in the above scan range may be reconstructed by helically projecting the data obtained in a helical form as above.
In the above conventional construction, as shown in FIG. 1, the X-ray tube 17 generates X-ray beams which diverge in a conical form from one focal spot F0 to enter the X-ray detector 12 (having five rows of detecting elements), thereby providing data. The helical scan having a coarse pitch would provide a large amount of data resulting from incident X-ray beams having data acquiring directions not perpendicular to Z-axis. An image reconstructed from such data is vulnerable to artifacts. Since the helical scan cannot have a large pitch as noted above, a time-consuming scanning operation is required to acquire data for a three-dimensional region having a length in the axial direction.